Recently, with the increase in public awareness of environmental issues such as global warming and others, expectations for solar power generation have been much raised in view of the cleanness thereof and freeness from environmental pollution. A solar cell constitutes the main part of a solar power generation system which directly converts the energy of sunlight into electricity. Regarding the configuration of the solar cell, multiple solar cell elements (simply referred to as cells) are connected in series or in parallel and are packaged for protecting the cells to give individual units. The unit installed in the package is referred to as a solar cell module, and, in general, this is so designed that the face thereof to be exposed to sunlight is covered with a transparent substrate (front sheet) of glass or the like as a top protective material and sealed up with a resin layer (encapsulant material) of a thermoplastic plastic (for example, ethylene/vinyl acetate copolymer), and the back thereof is protected with a back sealing sheet (back sheet) as a back protective material.
Those solar cell modules are mainly used outdoors, and are therefore required to satisfy various properties in point of the configuration, the material constitution and others thereof. Mainly the above-mentioned encapsulant material is required to have flexibility and impact resistance for protecting solar cell elements, heat resistance for protecting solar cell modules in heat generation, transparency (total light transmission, etc.) for efficient arrival of sunlight to solar cell elements, durability, dimensional stability, flame retardation, moisture-proofness, etc.
At present, as a encapsulant material for solar cell elements in solar cell modules, an ethylene/vinyl acetate copolymer (hereinafter this may be abbreviated as EVA) is widely used as the constituent material thereof, from the viewpoint of flexibility, transparency and the like (for example, see Patent Reference 1). Mainly for imparting heat resistance to EVA, crosslinking is attained with an organic peroxide serving as a crosslinking agent. For this, there is employed a method of previously preparing an EVA sheet mixed with a crosslinking agent (organic peroxide) and a crosslinking promoter and sealing up solar cell elements by the use of the resulting sheet.
However, in case where solar cell modules are produced by the use of the EVA sheet, EVA may thermally decompose to generate acetic acid gas thereby having some negative influences on the working environment and the production apparatus and bringing about some problems of circuit corrosion in solar cells, interlayer delamination of various parts such as solar cell elements, front sheet, back sheet and others, depending on various conditions in their production such as thermal bonding under pressure, etc.
Further, EVA is insufficient in moisture-proofness, concretely in water vapor barrier performance (for example, the water vapor permeability of EVA having a thickness of 0.3 mm at a temperature of 40° C. and a relative humidity of 90% is from 25 to 35 g/(m2·24 hours) or so), and therefore, in case of use in environments at high humidity or the like, moisture may reach the solar cell elements to be a cause of deteriorating the solar cells and lowering the power generation efficiency.
For solving the problems, for example, Patent Reference 2 discloses a solar cell encapsulant material using an ethylene/α-olefin copolymer. Patent Reference 3 discloses a solar cell encapsulant material using a silane-modified ethylene-based resin. As examples of resins more excellent in moisture-proofness than EVA, there are known high-density polyethylene and cyclic olefin-based resins with a cyclic olefin introduced into the molecular chain; and for example, Patent Reference 4 discloses a solar cell encapsulant material using a hydrogenated product of a norbomene-based ring-opened polymer as a cyclic olefin-based resin. Further, Patent Reference 5 discloses a solar cell encapsulant material of a laminate of an adhesive/heat-resistant layer and a buffer layer. Patent Reference 6 discloses a solar cell encapsulant material sheet produced by adding a C9-based aromatic hydrocarbon resin to EVA, though not intended to enhance moisture-proofness.